Conventionally, the chromatic aberration of an optical system has been corrected by combining optical elements made of glass materials with different dispersions. Use of a diffraction type optical system but not a refraction type optical system (lens) is disclosed in SPIE Vol. 1354, Pages 24 to 37.
When a diffraction surface with a diffraction efficiency is to be added to an optical system with spectroscopic characteristics, it is important to maintain high a diffraction efficiency in the wavelength band to be used. Regarding light of an order other than the designed order, the farther the order from the designed order, the larger the diffraction angle, and the larger the difference in focal length, which appears as defocus. When a particularly high-brightness light source is present, this defocus may sometimes be observed as a side lobe.
In an optical system having a chromatic aberration correcting effect, a diffraction optical element with a multilayered structure (obtained by stacking a convex lens type diffraction element and a concave lens type diffraction element) greatly improves a diffraction efficiency in the vicinity of a designed order within the band of a wavelength to be used. When this diffraction optical element is applied, a considerably large improvement can be expected in the image quality and information quality. Although the optical performance is improved as described above, when the conventional method is used, the manufacturing method becomes very complicated and very expensive, and is thus difficult to put into a practical use, as will be described hereinafter.
More specifically, regarding a method of realizing a diffraction optical element having a multilayered structure of two layers or more, photolithography as a semiconductor manufacturing process or a precision cutting technique by means of a diamond cutting tool is conventionally known as a typical micropatterning technique. Using a micropatterned shape formed by this processing technique as a mold, a plastic or glass can be molded, thereby manufacturing a diffraction optical element.
To achieve the above diffraction optical element with the multilayered structure, the diffraction gratings themselves to be stacked must be fabricated with high precision.
Conventionally, as a technique for manufacturing a duplicate of an optical element, a replica molding method using a photo-setting resin is preferred because of its transfer properties, surface precision, easiness, and the like. This method will be significant in the future as well because it can transfer a micropatterned shape obtained by photolithography. The transfer properties may be adversely affected by the setting shrinkage amount of the material itself, but various types of techniques have been proposed as this method. For example, Japanese Patent Laid-Open No. 3-79314 discloses a mold releasing technique. With a mold releasing method that causes turn-up when the depth of a micropattern becomes large, the following problems arise.
Since a multilayered diffraction optical element is obtained by literally bonding two diffraction optical elements, it must be thin in view of the weight or downsizing of the product. Conventionally, according to the replica molding method described above, the mold is released by pulling up a glass substrate larger than the mold diameter. During the progress of mold release, a very large warp (deformation) undesirably occurs in the entire element. For example, when a refractive index lens with a spherical or aspherical surface is to be molded, its molding shape may not be adversely affected by the replica molding method. In a diffraction optical element having a three-dimensional shape at a very small pitch on its surface, a mold release angle formed by warp or deformation serves as a force to cause the micropatterned molding shape to fall down, and deforms or damages it. The size of the warp is mainly determined by the adhesion properties between the material to be molded and the material of the mold. To decrease the adhesion properties, a mold release agent may be applied to the mold, but this is difficult when irregularity of the micropatterned shape and maintenance in mass production are considered.